In cellular systems, to further increase mobile network capacity, heterogeneous networks have been employed. An example of a typical heterogeneous network is shown in FIG. 1. The heterogeneous network of FIG. 1 comprises a macro base station, BS 11 covering a geographical area of a macro cell 12 and two pico BSs 21, 31 each covering a geographical area of a pico cell 22, 32. As seen in the figure, the coverage area of the macro cell is much larger than the coverage area of the pico cells. Consequently, the pico BSs communicate with a lower power than the macro BS. Further, the pico BSs are backhauled, i.e. they are each connected via a link 23, 33 to the mobile network, via the macro BS. The link may be a physical link, a micro wave link, a radio link etc.
Pico BSs may be deployed in small areas where traffic demand is high, such as indoor environment, for example in offices. In such high-demand areas it may be difficult to find fiber connections for the backhaul connection from the pico BS to a macro BS. To deploy fiber infrastructure to such pico BSs is both costly and time-consuming. Although, in many high-demand areas, for example in office buildings, there may be existing twisted pair wires, such as copper wires, for example DSL lines or Cat 5 cables. It would be advantageous if existing twisted pair wires could be reused for the backhaul connections between a pico BS and a macro BS.
Further, there are mainly two types of Heterogeneous network deployment of pico cells. The first type is called an independent pico cell deployment. In this case, the pico BS functions as a simplified macro BS with lower power and less base band processing capability. Consequently, in the independent pico cell deployment, the received signal is converted into the base band and at least partly processed in the pico BS such that only IP backhaul communication is required.
The second type of heterogeneous network deployment is called Remote Radio Unit-based (RRU-based) pico cells. Here, a pico BS only comprises a radio frequency subsystem, e.g. performing IQ sampling, and the RF signal is converted to a base band signal which is forwarded to a centralized base band unit in the macro BS. For RRU-based pico cells, an improved network performance is achieved since joint processing of signals received from several pico BSs can be achieved at the base band unit at the macro BS. According to prior art, the RF signal is converted to a base band signal which is forwarded to a base band unit in the macro BS via a Common Public Radio Interface (CPRI). In the base band unit the signal is processed and decoded. When using CPRI the RF signal is over-sampled and transferred in bits in the base band. This requires very high capacity, typically at least 1.25 Gbit/s and very low latency, typically less than 250 μs. Also, even if there is no data to transfer, the over-sampling has to be performed. Due to the high capacity requirements and the low latency requirements, the CPRI is not possible, or at least disadvantageous, to use for transferring signals over a twisted-pair wire between an RRU-based pico BS and a macro BS. Consequently, there is a need for another solution for transferring signals between a pico BS and a macro BS, especially for communication between an RRU-based pico BS and a macro BS.